Why disk drives have become slower over the years

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What is the first question vendors get (or at least used to get) when a customer (non-technical) calls???
I’ll spare you the guesswork: “What does a TB of disks do at your place ??”. Usually I’ll goofle around for the cheapest disk at a local PC store and say “Well Sir, that would be about 80 dollars”. I then hear somebody falling of the chair, trying to get up again, reach for the phone and with a resonating voice asking “Why are your competitors so expensive then?”. “They most likely did not gave a direct answer to your question.”, I reply.
The thing is an HDD should be evaluated on multiple factors and when you spend 80 bucks on a 1TB disk you get capacity and that’s about it. Don’t expect performance or extended MTBF figures let alone all the stuff than comes with enterprise arrays like large caches, redundancy in every sense and a lot more. This is what makes up the price per GB.


“Ok, so why have disk drives become so much slower in the past couple of years?”. Well, they haven’t. The RPM, seek time and latency have remained the same over the last couple of years. The problem is that the capacity has increased so much that the so called “access density” has increased linearly so the disk has to service a massive amount of bytes with the same nominal IOPS capability.


I did some simple calculations which shows the decrease in performance on larger disks. I didn’t assume any raid or cache accelerators.


I first calculated a baseline based on a 100GB disk drive (I know, they don’t exist but it just for the calculations) with 500GB of data that I need to read or write.


The assumption was to have a 100% random read profile. Although the host can read or write in increments of 512 bytes IO size theoretically this doesn’t mean the disk will write this IO in one sequential stroke. An 8K host IO can be split up in the smallest supported sector size on disk which is currently around 512 bytes. (Don’t worry, every disk and array will optimize this but again this is just to show the nominal differences)


So when I have 100GB disk drive this translates to a little over 190 million sectors.  In order to read 500 GB of data this would take a theoretical 21.7 minutes. The number of disks are calculated based on the capacity required for that 500GB (Also remember that disks use a base10 capacity value whereas operating systems,memory chips and other electronics use a base2 value so that’s 10^3 vs 2^10.)

Baseline
Sectors RPM Avrg delay in ms Max IOPS Disks required 6
100 190,734,863 10000 8 125 Num IOPS 750
Time Required 1302
in minutes 21.7

 If you now take this baseline and map this to some previous and current disk types and capacities you can see the differences.

GB Sectors RPM # Disk per A29 Num IOPS Time required in sec in min %pcnt of base line * times base value
9 17,166,138 7200 57 4731 206 3.44 15.83 6.32
18 34,332,275 7200 29 2407 406 6.77 31.19 3.21
36 68,664,551 10000 15 1875 521 8.69 40.02 2.50
72 137,329,102 10000 8 1000 977 16.29 75.04 1.33
146 278,472,900 10000 4 500 1953 32.55 150 0.67
300 572,204,590 10000 2 250 3906 65.1 300 0.33
450 858,306,885 10000 2 250 3906 65.1 300 0.33
600 1,144,409,180 10000 1 125 7813 130.22 600.08 0.17

You can see here that capacity wise to store the same 500GB on 146 GB disks you need less disks but you also get fewer total IOPS. This then translates into slower performance. As an example a 300GB drive with 10000RPM triples the time compared to the baseline disk to read this 500 gigabyte.


Now these a re relatively simple calculations however they do apply to all disks including the ones in your disk array.


I hope this also makes you start thinking about performance as well as capacity. I’m pretty sure your business finds it most annoying when your users need to get a cup of coffee after every database query. 🙂

About Erwin van Londen

Master Technical Analyst at Hitachi Data Systems
Storage Networking , , , , ,
  • SSD will indeed solve the performance factor but now they have to work on capacity as well as reliability.

    I modified the font size. Looks like Goofle adjusts it to the smallest possible to be able to include the tables as they should look.

  • Very interesting and timely…. leads to the question of SSDs..

    Only complaint is that the font is coming out really small. Not sure if its the browser or WordPress? Or maybe your eyes are better than mine…. #;-)